1. Field of the Invention
The present invention relates to electrodeposition compositions and, more particularly, to aqueous dispersions of electrodeposition compositions containing capped polyisocyanate curing agents.
2. Brief Description of the Prior Art
Electrodeposition compositions, particularly cationic electrodeposition compositions containing capped polyisocyanate curing agents, are known in the art. The cationic compositions, particularly those derived from cationic epoxy-amine reaction products, provide for outstanding corrosion resistance over steel substrates and have been used extensively as primers for automobile and truck bodies. A particularly desirable polyisocyanate for use in these compositions is diphenylmethane-4,4'-diisocyanate, commonly called MDI, and a mixture of diphenylmethane-4,4'-diisocyanate and polymethylene polyphenyl isocyanate, commonly referred to as crude MDI. Such polyisocyanates are relatively non-volatile at curing temperatures compared to other aromatic polyisocyanates such as toluene diisocyanate, and this lack of volatility is a distinct advantage in primer applications where the volatile polyisocyanates can discolor and impede the cure of the subsequently applied top coats. This problem is discussed in U.S. Pat. No. 4,296,010.
A disadvantage associated with MDI and crude MDI is that these polyisocyanates often result in unstable electrodeposition compositions. The compositions are in the form of aqueous dispersions and it has been found that the resinous phase often sediments from the dispersion. The problem is particularly acute when the polyisocyanate is capped with a low molecular weight capping agent such as methanol or ethanol. The sediment cannot easily be redispersed and results in an electrodeposition bath which cannot be serviced adequately and which does not provide for satisfactory electrodeposited coatings.
Surprisingly, it has been found that particular mixtures of a diphenyl-2,4'-diisocyanate and a diphenyl-4,4'-diisocyanate provide for capped isocyanates which, when formulated into aqueously dispersed electrocoating compositions, provide for stable dispersions and satisfactory electrodeposited coatings.